This invention relates to integrity testing for filtering membranes, particularly for suction driven microfiltration or ultrafiltration membranes.
One advantage of using membranes to filter water is that membranes are able to remove very small particles including pathogenic microorganisms and colloids. Thus, strong chemicals may not be required as a primary disinfectant in drinking water applications and a nearly complete lack of colloids in water improves the performance of many industrial processes.
To ensure that undesired particles are removed, the integrity of a membrane unit must be tested. This is often done by air leak tests (pressure hold tests and pressure decay tests) performed while permeation is temporarily stopped. Although effective, each test interrupts permeation and lowers the yield of the process. Accordingly, discontinuous testing is typically performed at most every 4 hours and a leak could go un-noticed for up to that long. Thus, at least the California Department of Health has indicated that it requires continuous integrity testing before chlorination requirements can be further reduced for membrane filtration plants.
The dominant continuous integrity testing techniques involve in-line instruments such particle counters, particle monitors or turbidity meters. These instruments do not evaluate the membrane itself but instead monitor and assess a surrogate parameter to diagnose the membrane condition. For instance, a particle counter generally includes a light scattering sensor, typically laser-based, interfaced with a computer running particle enumeration software that assesses the number of particles in one or more particle size ranges: see generally Panglish et al., xe2x80x9cMonitoring the Integrity of Capillary Membranes by Particle Countersxe2x80x9d, Desalination, vol. 119, p. 65-72 (1998). Similarly, a particle monitor that measures the fluctuation in intensity in a narrow light beam transmitted through a permeate sample is also known. Through subsequent computer analysis, the observed fluctuations can be converted into an index of water quality.
Such in-line testing equipment is elaborate and expensive. Unfortunately, the number of membrane units or modules that can be simultaneously monitored using a single equipment set-up is limited by dilution effects. For example, a defect such as a broken fibre in a large (ie.  greater than 1 MGD) plant could cause a health concern but may not be detected by in-line testing equipment sampling the plant outlet.
One solution that has been proposed for pressure driven membranes is the MEDUSA(trademark) system by Hach Company. In the MEDUSA(trademark) system, a monitoring line is tapped into the permeate collector of each of several small membrane assemblies upstream of where those permeate collectors join the main permeate outlet pipe. A portion of the pressurized permeate flows through the monitoring line which passes through a turbidimeter body and then to a drain. Each of the turbidimeter bodies is connected to a central laser multiplexer and a detector multiplexer which are in turn connected to control electronics. Thus, each of a plurality of small pressure driven membrane assemblies are monitored individually.
An object of the present invention is to provide a method and/or apparatus for providing a continuous integrity test for filtering membranes, particularly suction driven microfiltration or ultrafiltration membranes. The inventors have observed that with suction driven membranes, pressurized permeate as required by the prior art is only available downstream of the permeate pump. Economical large plant design, however, requires a single permeate pump for several distinct membrane assemblies. The dilution effect discussed above prevents in-line testing equipment from detecting a leak in a fibre by sampling permeate downstream of the permeate pump in a large plant.
In one aspect of the present invention, a monitoring line carries permeate from a membrane assembly through an in-line monitoring device. The membrane assembly is small enough, in view of the dilution effect, to be adequately tested by the in-line monitoring device. Both ends of the monitoring line are connected to an assembly permeate pipe of the membrane assembly. To produce a flow of permeate through the monitoring line, its inlet and outlet are placed at points of relatively higher and lower pressures respectively. Preferably, each end of the monitoring line is placed on the opposite sides of a pressure drop in the assembly permeate pipe. The pressure drop is greater than the pressure drop through the in-line monitoring device and the monitoring line. The pressure drop can be created by existing components, such as a series of bends in the piping or an existing valve, or from an added component such as an orifice plate or an additional valve.
By using a pressure drop in a permeate pipe to generate a flow of permeate for the in-line monitoring device, permeate can be monitored in a suction-driven system before it is pressurised by the permeate pump. This allows membrane assemblies to be individually monitored by connecting monitoring lines to assembly permeate pipes associated with each of a plurality of membrane assemblies even though one permeate pump services all of them, for example through a header.